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XII. Depth Related Patterns

Depth Related Patterns

We also see that different types of marine animals exhibit different depth-related patterns. Visually-cued predators like fishes and crustaceans show a rapid decrease in metabolism and muscle enzymatic activity with depth. Gelatinous species, including jellyfish and ctenophores, which do not depend on their visual sense for predation, do not show a decrease in metabolic rate or enzymatic activity with depth.

Recent work on gelatinous organisms by Eric Thuesen and Jim Childress of the University of California, Santa Barbara, has shown clearly how different the depth-related patterns in respiration are between visual and non-visual predators.

I've emphasized that if we measure respiration rate, as well as enzymatic activity, we see a strong correlation. I want to take a side trip here and talk about the uses to which biochemical methods can be put to get a handle on the physiological status of natural populations. Here is the logic. We've seen that enzymatic activities can provide an index of oxygen consumption rates. These in turn are an index of physiological status. In other words, if we measure the rate of oxygen consumption of an organism and plot that against the amount of ATP generating activity in a gram of muscle, we see a very strong correlation. But what if we want to get some sense of the physiological status of fish that we can't keep alive for respiration measurements? Let's say we wanted to get an estimate of respiration rate for deep sea fishes that are very difficult to bring up alive. As mentioned earlier, a lot of these organisms are very fragile. They're difficult to bring up in good shape. We have to get a lot of our evidence about what these organisms are like through indirect methods. Video measurements are wonderful for some purposes; indirect biochemical analysis work very well, too. If we know how respiration rate and muscle enzymatic activity co-vary, we can simply measure enzymatic activity and use this value to predict the rate of oxygen consumption.

One organism that we've used in studies that employ enzymatic indices of physiological state is Sebastolobus alascanus, known as the idiot fish. This photo taken on the sea floor shows this fish in its normal habitat. It's a commercially important species that occurs at a number of depths and is found within the oxygen minimum zone. We collected a number of these fish using set lines and held the fish for several months in the laboratory. Some were fed; some were starved. What we observed was that the fed fish had higher rates of respiration and higher levels of enzymatic activity in their muscles than the starved fish. Furthermore, when we compared freshly-collected fish from the field with the lab-maintained fish, we found evidence that the field fish were living under food-limiting conditions. Both respiration rate and enzymatic activity in the field-collected fish were similar to values obtained for starved fish. These data suggest that, down in the oxygen minimum zone, food may be very limiting for this species, at least at certain times and in certain places. With this "calibration curve" between respiration rate and enzymatic activity, it should now be possible to sample hundreds or thousands of these fish from different regions to get a sense of how well they are doing physiologically. Rapid and inexpensive measurements of enzymatic activity should provide strong evidence about the nutritional status and respiration rates of the specimens. These data could be of some value to fisheries scientists who are concerned about the status of commercially important species.


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